ProtozoaAmoebae
Entamoeba histolytica
- Epidemiology: Worldwide; tropics, poor sanitation.
- Transmission: Ingestion of cysts (faeco-oral).
- Clinical: Amoebic dysentery; amoebic liver abscess ("anchovy sauce pus"); rare pulmonary/brain spread.
- Diagnosis:
- Microscopy (trophozoites with ingested RBCs).
- PCR/antigen (distinguishes from E. dispar).
- Serology (useful in liver abscess).
- Imaging (US/CT) helpful in detecting liver abscess.
- Treatment:
- Metronidazole + luminal agent (paromomycin/diloxanide).
- Drain abscess if risk of rupture or poor response to therapy;

Acanthamoeba spp.
- Epidemiology: Free-living amoeba in soil, water, contact lens solutions.
- Transmission: Traumatic inoculation into cornea; inhalation/cutaneous entry → CNS (immunocompromised).
- Clinical:
- Keratitis: severe, painful, ring infiltrates in contact lens wearers; often misdiagnosed as HSV keratitis.
- Granulomatous amoebic encephalitis (GAE): subacute/chronic in immunocompromised; weeks–months.
- Diagnosis: Corneal scrapings: culture on non-nutrient agar with E. coli overlay; PCR; confocal microscopy.
- Treatment:
- Keratitis: prolonged topical therapy (biguanides, diamidines, ± azoles).
- GAE: multi-drug regimens (miltefosine, pentamidine, azoles, flucytosine, macrolides). Poor prognosis.

Naegleria fowleri
- Epidemiology: Free-living; warm freshwater lakes/rivers, poorly chlorinated pools. Rare but fulminant.
- Transmission: Water forced into nasal passages → olfactory mucosa → CNS. Not by ingestion.
- Clinical: Primary amoebic meningoencephalitis (PAM): acute, rapidly progressive; death in days.
- Diagnosis: CSF: neutrophilic pleocytosis, ↓ glucose, ↑ protein; wet mount: motile trophozoites (distinguishes from Acanthamoeba); PCR confirmation. Culture possible on non-nutrient agar with bacteria, though rarely used.
- Treatment: Amphotericin B (IV + intrathecal), rifampicin, azoles, miltefosine; early aggressive therapy + hypothermia sometimes successful. Mortality >95%.
- Prevention: Avoid nasal exposure to warm freshwater; proper pool chlorination.

Balamuthia mandrillaris
- Epidemiology: Rare; soil exposure.
- Transmission: Inhalation or skin → CNS.
- Clinical: Granulomatous amoebic encephalitis; cutaneous lesions.
- Diagnosis: Histology (amoebic trophozoites and cysts), PCR.
- Treatment: Multi-drug regimens (miltefosine, azoles, macrolides, flucytosine); prognosis poor.

Intestinal Protozoa
Giardia lamblia (intestinalis, duodenalis)
- Epidemiology: Worldwide; waterborne outbreaks, esp. travellers/children.
- Transmission: Ingestion of cysts from contaminated water/food.
- Clinical: Malabsorptive diarrhoea, bloating, steatorrhoea; chronic infection → growth failure in children.
- Diagnosis: Stool microscopy (trophozoite with “falling leaf motility”); antigen detection ELISA, PCR; string test (Entero-test) is a classical though rarely used method.
- Treatment: Metronidazole, tinidazole, or nitazoxanide.

Cryptosporidium (C. hominis, C. parvum)
- Epidemiology: Worldwide; zoonotic reservoirs. Oocysts chlorine-resistant → waterborne outbreaks.
- Transmission: Ingestion of oocysts (faeco-oral, contaminated water, animal contact). Oocysts are immediately infectious on excretion.
- Clinical:
- Immunocompetent: watery diarrhoea, self-limited.
- Immunocompromised: chronic diarrhoea, biliary/pancreatic involvement.
- Diagnosis: Modified Ziehl–Neelsen (acid-fast oocysts 4–6 μm); antigen assays; PCR.
- Treatment: Supportive; nitazoxanide (variable efficacy); optimise ART in HIV.
- Infection control: Strict hygiene, water filtration; chlorine insufficient.

Cyclospora cayetanensis
- Epidemiology: Tropics/subtropics; foodborne outbreaks (berries, salad).
- Transmission: Ingestion of oocysts (require sporulation outside host to become infectious).
- Clinical: Relapsing/prolonged watery diarrhoea, esp. in travellers or immunocompromised.
- Diagnosis: Variable acid-fast oocysts (8–10 µm, larger than Cryptosporidium); autofluorescence; PCR.
- Treatment: Co-trimoxazole.

Cystoisospora (Isospora) belli
- Epidemiology: Rare; immunocompromised (HIV).
- Transmission: Ingestion of oocysts (require sporulation outside host to become infectious).
- Clinical: Watery diarrhoea, malabsorption, weight loss.
- Diagnosis: Large acid-fast oocysts (20–30 µm, larger than Cryptosporidium and Cyclospora); PCR.
- Treatment: Co-trimoxazole.
Balantidium coli
- Epidemiology: Rare; zoonosis (pigs).
- Transmission: Ingestion of cysts (faeco-oral).
- Clinical: Dysentery-like illness; colitis.
- Diagnosis: Large ciliated trophozoites in stool.
- Treatment: Tetracycline; alternatives: metronidazole, iodoquinol.

Urogenital Protozoa
Trichomonas vaginalis
- Epidemiology: Worldwide; common STI.
- Transmission: Sexual contact.
- Clinical:
- Women: vaginitis, frothy discharge, “strawberry cervix.”
- Men: urethritis, prostatitis (often asymptomatic).
- Diagnosis: Wet mount (motile trophozoites, “jerky motility”); antigen/PCR.
- Treatment: Metronidazole or tinidazole (treat partners).

Blood & Tissue Protozoa
Plasmodium spp. (falciparum, vivax, ovale, malariae, knowlesi)
- Epidemiology: Endemic in tropics/subtropics.
- Transmission: Anopheles mosquito.
- Clinical: Malaria: cyclical fever, anaemia, splenomegaly; falciparum → severe malaria (cerebral, renal failure, acidosis). Vivax and ovale can relapse due to dormant hypnozoites in liver.
- Diagnosis: Thick/thin blood films (species ID); rapid diagnostic tests; PCR.
- Treatment:
- Uncomplicated: artemisinin-based combination therapy (ACT).
- Severe: IV artesunate.
- Primaquine required to eradicate hypnozoites (vivax/ovale).
- Prevention: Chemoprophylaxis, vector control.
Babesia (B. microti, B. divergens)
- Epidemiology: Tick-borne (Ixodes). B. microti (US); B. divergens (Europe, severe in splenectomised).
- Transmission: Tick bite; transfusion; rarely congenital.
- Clinical: Malaria-like fever, haemolysis; severe in splenectomised, elderly, immunocompromised.
- Diagnosis: Blood film: intraerythrocytic rings, Maltese cross tetrads; lacks haemozoin pigment (unlike malaria). PCR; serology.
- Treatment:
- Mild: atovaquone + azithromycin.
- Severe: clindamycin + quinine; exchange transfusion if high parasitaemia.
- Prevention: Tick avoidance; donor blood screening.

Trypanosoma brucei (gambiense, rhodesiense)
- Epidemiology: Sub-Saharan Africa; vector: tsetse fly (Glossina).
- Transmission: Bite of infected fly.
- Clinical: African trypanosomiasis (“sleeping sickness”):
- Chancre at bite site.
- Haemolymphatic stage: fever, lymphadenopathy (Winterbottom’s sign).
- CNS stage: somnolence, confusion, coma.
- Diagnosis: Blood/CSF microscopy (trypomastigotes); serology (CATT test for T. b. gambiense).
- Treatment:
- New: Fexinidazole is an oral nitroimidazole recently licensed for all stages of T. b. gambiense sleeping sickness.
| gambiense | rhodesiense |
Early | pentamidine | suramin |
CNS | eflornithine ± nifurtimox | melarsoprol |
- Additional note: Pathogenesis involves antigenic variation of surface glycoproteins.
- Key differences:
- T. b. gambiense: West/Central Africa, chronic disease (months–years), human reservoir, more common; CNS involvement late; better prognosis.
- T. b. rhodesiense: East/Southern Africa, acute/rapid course (weeks), zoonotic reservoir (cattle, game), more severe, early CNS involvement; high mortality if untreated.

Trypanosoma cruzi
- Epidemiology: Latin America; vector: reduviid “kissing bug.”
- Transmission: Contamination of bite by vector faeces; transfusion; congenital.
- Clinical:
- Acute: fever, chagoma, Romana’s sign (periorbital oedema).
- Chronic: cardiomyopathy, arrhythmia, megaoesophagus, megacolon.
- Diagnosis: Acute: blood film (trypomastigotes). Chronic: serology, PCR. Xenodiagnosis is a traditional though rarely used method.
- Treatment: Benznidazole or nifurtimox (more effective in acute).
Leishmania spp.
- Epidemiology: Mediterranean, Middle East, Asia, South America; vector: sandfly.
- Transmission: Bite of infected sandfly.
- Clinical:
- Visceral leishmaniasis (kala-azar): fever, weight loss, hepatosplenomegaly, pancytopenia.
- Cutaneous leishmaniasis: localised skin ulcers at bite site.
- Mucocutaneous leishmaniasis: destructive mucosal lesions (nasopharyngeal).
- Diagnosis: Amastigotes (Leishman-Donovan bodies) in macrophages; PCR; serology; rk39 antigen test used for visceral disease.
- Treatment:
- Visceral leishmaniasis: liposomal amphotericin B (first-line); alternatives: miltefosine, pentavalent antimonials (resistance in India).
- Cutaneous leishmaniasis: often self-limiting; local therapies (cryotherapy, intralesional antimonials); systemic treatment (miltefosine, amphotericin B) for severe, disseminated, or mucocutaneous disease.

Zoonotic / Opportunistic Protozoa
Toxoplasma gondii
- Epidemiology: Worldwide; cats are definitive hosts.
- Transmission: Ingestion of oocysts (cat faeces) or tissue cysts (undercooked meat); congenital.
- Clinical:
- Immunocompetent: mild flu-like, lymphadenopathy.
- HIV: encephalitis (multiple ring-enhancing brain lesions).
- Congenital: triad (chorioretinitis, hydrocephalus, intracranial calcifications).
- Diagnosis: Serology (IgM/IgG, avidity); PCR (blood, CSF); imaging in HIV.
- Treatment: Pyrimethamine + sulfadiazine + folinic acid. Spiramycin in pregnancy to reduce congenital transmission.

Microsporidia (Enterocytozoon bieneusi, Encephalitozoon spp.)
- Epidemiology: Opportunistic, esp. HIV/immunosuppressed.
- Transmission: Ingestion or inhalation of spores.
- Clinical: Chronic diarrhoea, wasting; keratitis; disseminated disease.
- Diagnosis: Modified trichrome stain; EM; PCR. Spores are very small (1–2 μm).
- Treatment: Albendazole (variable species activity); ART in HIV.

Key Buzzwords for Exams
- Entamoeba: “anchovy sauce pus.”
- Giardia: “falling leaf motility.”
- Cryptosporidium: “acid-fast oocysts, swimming pools.”
- Cyclospora: “autofluorescent oocysts.”
- Cystoisospora: “large acid-fast oocysts.”
- Balantidium: “large ciliate trophozoite.”
- Trichomonas: “strawberry cervix.”
- Plasmodium falciparum: “banana-shaped gametocyte.”
- Babesia: “Maltese cross; no pigment.”
- T. brucei: “Winterbottom’s sign; antigenic variation.”
- T. cruzi: “Romana’s sign.”
- Leishmania: “amastigotes in macrophages.”
- Naegleria: “motile trophozoites in wet CSF.”
- Acanthamoeba: “ring infiltrate keratitis.”
- Toxoplasma: “ring-enhancing brain lesions in HIV; spiramycin in pregnancy.”
- Microsporidia: “tiny 1–2 μm spores.”